Author's Note: The following report has not been subjected to the scientific peer review process.

1. Introduction

Snow fell across the mountains of the western Carolinas most of the night
Saturday, 18 November 2000, and into the day on Sunday 19 November. An
accumulating late November snow is not that unusual in the mountains. What
happened in the foothills and piedmont, however, was quite unusual for the
region. During the late morning hours on 19 November, the rain/snow line
began to advance east across the foothills and into the piedmont. By late
morning, most of the Greenville-Spartanburg County Warning Area was
experiencing snow. Snow fell moderately over this area until mid afternoon,
when the precipitation ended abruptly from west to east, the result of
subsidence around the upper jet. Snowfall totals averaged from 2 to 5 inches
over the mountains (a bit more in spots), and from a trace to 3 inches east
of the mountains (Fig. 1).

Figure 1. Total snow accumulation for the 18 November 2000 Winter Storm.
Click on image to enlarge.

The 2.5 inches of snow recorded at the Greenville-Spartanburg Airport for
the day was the heaviest snowfall ever recorded in November, beating the
old record of 1.9 inches set back in 1968. For additional November snow
data visit this link: Greenville-Spartanburg November Precipitation Records.
The record for the day was tied at Charlotte Douglas International Airport
where 2.5 inches of snow also fell. The previous record was also set in 1968.
The Asheville Regional Airport recorded 2 inches of snow for the day, quite
a bit less than other mountain locations.

2. Discussion

Why the surprise snow? Without getting too technical, the lower levels of
the atmosphere were expected to be too warm for snow east of the mountains.
And for much of the morning, this was the case, with only some occasional
mix-over to sleet. Two cooling processes, however, conspired to lower the
temperatures in the lower atmosphere enough to support snow. One process
was evaporational cooling. This occurs when rain evaporates as it falls
through a dry air mass. This alone was probably not enough to cause the
transition to snow. The other process was the melting of snow. As snow
falls and melts, it also robs the atmosphere of heat, just as rain does
when it evaporates. This process, however, can occur even in a saturated
atmosphere, unlike evaporative cooling. Based on the precipitation rates
observed late Sunday morning, it is estimated that melting of snow resulted
in about 2 degrees Celsius of cooling. This cooling process was likely what
lead to the Interstate-85 corridor snows.

An examination of critical thickness for freezing and frozen precipitation
revealed that every line except for one was northwest of the foothills and
piedmont (Fig. 2). Generally, locations northwest of these lines
(particularly the yellow and turquoise lines) will experience snow and
locations to the south will experience rain. This data was taken from the
morning run of one of our best numerical models, and is valid for 2 pm
Sunday afternoon, the height of the snow event east of the mountains. As
computational power increases, the models will be better able to resolve
cooling due the melting of snow.

Figure 2. Critical thickness lines from the numerical models on
18 November 2000. Image courtesy of The College of DuPage.

This Public Information Statement provides detailed information of snowfall
across the Greenville-Spartanburg County Warning Area. Note though that
some of these totals reflected the amount on the ground by late evening,
and some reflected estimated storm total snowfall.